In the parent applications of the current application measurements are described of absorption spectra from sub-micron regions of a sample surface, using a tunable infrared source to illuminate a region of the sample while recording the absorption of IR radiation with a cantilever probe of a probe microscope. The various techniques associated with these measurements are known as AFM-IR. One technique previously disclosed is to modulate or pulse the tunable infrared source at a frequency corresponding to a resonance of the microscope probe, typically a cantilever. In one embodiment, the infrared source is pulsed at a contact resonant frequency of the cantilever. Useful results have been demonstrated employing this technique by Mikhail Belkin and colleagues; F. Lu and M. A. Belkin, “Infrared absorption nano-spectroscopy using sample photoexpansion induced by tunable quantum cascade lasers,” Opt. Express, in press (2011).
The Inventors have observed however that for some probe microscope configurations and/or samples of interest, the resonant frequency, especially the contact resonant frequency can vary over time and/or over different regions of the sample. The contact resonant frequency can change, for example, due to elastic and dissipative variations in the sample, topographic variations that give rise to different tip-surface contact area, changes in tip-surface interaction force, changes in lateral or longitudinal stresses on the cantilever, and other issues.
It is the object of the current invention to extend the techniques of modulating the IR source at or near a probe resonant frequency, including contact resonant frequencies, to the cases where the resonant frequency shifts significantly over time or relative positioning on the sample.